Volume 9, Number 4 • July/August 2001 • Aerospace Technology Development

Experiment Demonstrates Improved Test Method

The Aerostructures Test Wing (ATW), which consisted of an 18-inch carbon fiber test wing with surface-mounted piezoelectric strain actuators, following intentional failure on its final flight. Photo provided by NASA Dryden Flight Research Center.

A flight experiment called the Aero-structures Test Wing (ATW), conducted at NASA’s Dryden Flight Research Center in Edwards, California, successfully demonstrated a new software data analysis tool, the flutterometer, which is designed to increase the efficiency of flight flutter testing.

The experiment consisted of an 18-inch carbon fiber test wing with surface-mounted piezoelectric strain actuators. The test wing was mounted on a special ventral flight test fixture and flown on Dryden’s F-15B Research Testbed aircraft.

Five flights consisted of increasing speeds and altitudes leading to the final test point of Mach .85 at an altitude of 10,000 feet. At each Mach and altitude, stability estimations of the wing were made using accelerometer measurements in response to the piezoelectric actuator excitation. The test wing was intentionally flown to the point of structural failure, resulting in about a third of the 18-inch wing breaking off. This allowed engineers to record the effectiveness of the flutterometer over the entire regime of flutter testing, up to and including structural failure.

The actuators were moved at different magnitudes and frequency levels to induce wing vibrations and excite the dynamics during flight. Placement of the piezoelectric actuators was determined by NASA Langley Research Center, Hampton, Virginia Engineer Mercedes Reaves to maximize their effectiveness. The ATW experiment represents the first time that piezoelectric actuators were used during a flight flutter test. Flight flutter testing is the process of determining a flight envelope within which an aircraft is safe to operate. Traditional approaches for flight flutter testing do not accurately predict the onset of instability; so this testing is a very time-consuming and expensive process.

“The data acquired during the Aerostructures Test Wing experiment will help us to improve the way we model structures and to validate the flutterometer concept,” said Dryden Project Engineer David Voracek. “The data that was created from these flights will be invaluable to future flight flutter test engineers for research and training.”

Potential benefits of this research include reduced time and cost associated with aircraft certification by lowering the number of flights required to clear a new or modified aircraft for flight, and provision of a structural dynamics database for industry and university flutter research.

The flutterometer is an online software tool that was loaded on computers in Dryden’s control room for the experiment in order to access the flight data. With this new technology, flight data can be analyzed immediately using the newly developed software to determine the stability properties of aircraft in flight. It is designed to predict the flight conditions at which the onset of flutter may occur. In this way, the flight envelope of an aircraft can be determined more quickly and safely than using traditional approaches. The ATW experiment was the first time the flutterometer was used on a flight system that actually experienced flutter.

NASA was recently awarded a patent for the flutterometer. Its software program combines the strengths of analytical predictions and online estimation methods in the development of a flutterometer concept. The flutterometer software has previously been evaluated using simulations and wind tunnels, along with flight data from several aircraft types including NASA Dryden’s F-18 Systems Research Aircraft.

Flutter is the rapid and self-excited vibration of wings, tail surfaces and other aircraft parts that can damage or destroy an aircraft component. Flutter is caused by the flow of air across the surface of the structure. Effectively, the aerodynamic forces couple with the structural bending and twisting to result in the destructive vibration.

“The flutterometer represents a significant advance for flight flutter testing,” said Dryden Project Engineer Rick Lind. “This tool can result in dramatic decreases in time and cost for military and commercial aircraft testing. The ATW experiment was a perfect demonstration of how the unique facilities at NASA Dryden can be used to develop tools that are beneficial to the entire aviation industry.”

The ATW was designed by NASA Engineer Cliff Sticht and was manufactured by Fiberset, Inc., located in Mojave, California. Q

For more information, contact Rick Lind at Dryden Flight Research Center, & 661/276-3075, ) rick.lind@mail.dfrc.nasa.gov or David Voracek at Dryden Flight Research Center, & 661/276-2463, ) david.voracek!@mail.dfrc.nasa.gov Please mention you read about it in Innovation.

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